The use of ultrasound beams for non invasively detecting velocity information of moving reflectors in a body under study is a well known technique. Several alternative ways are known and currently used for determining the velocity of a scatterer from the frequency or phase shift, which affects a back-scattered ultrasound beam according to the Doppler effect.
One of these methods is the so called Multigate Doppler processing method, which includes the steps of:                a) transmitting ultrasound waves into the subject under study;        b) generating back-scattered signals in response to the ultrasound waves back-scattered from the subject under study;        c) generating a plurality of Doppler signal samples representing a predetermined range of depth increments within said subject in response to said back-scattered signals;        d) generating a plurality of Doppler frequency signals representing said predetermined range of depth increments in response to said Doppler signal samples; and        e) displaying a first Doppler graph representing said Doppler frequency along a first axis and said range of depth increments along a second axis in response to said Doppler frequency signals.        
In particular, Multigate Doppler processing is a PW Doppler technique, which allows dividing in a predefined range several smaller sample volumes corresponding to a certain number of successive depths increments along a transmitted beam emitted toward a bigger sample volume (gate) within a subject under study, in which Doppler frequency shift profile has to be determined. Doppler frequency profiles as a function of said depths increments means the velocity profiles of moving particles within said succession of smaller sample volumes. The processing of the Doppler signal samples, backscattered from the subject at each of said smaller samples volumes, i.e. the depths increments, is carried out essentially in parallel.
The above equivalence between sample volumes and depth increment along a transmitted beam is due to the fact that in order to acquire Doppler data with a PW technique, a pulsed ultrasound beam has to propagate along a direction, preferably at an angle related to the direction of motion of the reflectors.
A number of other different methods for ultrasonic detection of hemodynamic information are known and widely used in the art, and allow the determination of the average Doppler shift frequency and hence the average blood flow velocity in a predetermined point, or the spectral representation of Doppler shifts in a predetermined point, and show the distribution of the velocities of the blood particles of said flow in said point.
A method commonly known as CFM (Color Flow Mapping), which is used for determining the average Doppler shift frequency and hence the average velocity of a blood flow, consists in determining, for a predetermined point at one vessel, the mean of the spectral distribution of Doppler shift frequencies of ultrasonic pulses in said point. As an acquisition and processing method, CFM is known and widely used. See for instance U.S. Pat. No. 5,246,006. The ultrasonic signals are transmitted, received and processed to detect, for predetermined points along a predetermined scan line, the average spectrum frequency value in said point. Said average frequency is an estimate of the average displacement velocity of the reflector that moves through said point and hence of blood flowing through said point.
The visual result of the CFM method includes indication of the flow direction by one of two different colors, each being uniquely associated with one of the two directions, towards and away from the probe. Furthermore, the hue of said color indicates the intensity of the signal and hence the flow and/or the modulus of the average velocity.
As a rule, at the same time as Doppler processing of ultrasonic signals, a morphological (anatomic) image is also generated along a scan plane in the so-called B-Mode, particularly along a scan plane that contains the scan line/s used for CFM detection. The signals required for generation of the B-mode image are generally transmitted, received and processed alternating with transmission, reception and processing of Doppler signals.
Colors are added to the pixels of the B-mode image that coincide with the area or point at which the Doppler frequency shift has been detected.
Since a pulse of finite length is transmitted, the pulse will have a specific bandwidth and not only the fundamental frequency. Furthermore, within each sample volume (gate), several different kinds of moving reflectors can be provided so that the backscattered wave have a specific bandwidth and a specific spectral distribution of the frequencies within said bandwidth.
In addition to determining the average velocity value, i.e. the component of the mean frequency of the spectrum of Doppler frequency shifts, Doppler techniques are known, in which the entire spectrum of Doppler frequencies associated with a given sample point or volume is extracted. As already indicated above, these methods, known as Pulsed Wave (PW) methods, require more complex processing of ultrasonic signals, due to the much larger amount of information to be processed in comparison to CFM. An extension of the PW technology is the so-called Multigate which includes sequential Doppler spectrum detection through multiple sequential points (known as gates), arranged along a scan line, or a part of it, for reconstructing the velocity profile along said line.
The Multigate method is described in detail in the following documents:    P. Tortoli, G. Manes, C. Atzeni, Velocity profile reconstruction using ultrafast spectral analysis of Doppler ultrasound, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-32, N. 4, pp. 555—561, July 1985;    P. Tortoli, F. Andreuccetti, G. Manes, C. Atzeni, Blood Flow Images by a SAW-Based Multigate Doppler system, IEEE Transactions on Ultrasonics, Ferroelectrics & Frequency Control, vol. 35, n. 5, pp. 545-551, September 1988;    P. Tortoli, F. Guidi, G. Guidi, C. Atzeni, Spectral velocity profiles for detailed ultrasound flow analysis, IEEE Trans. on Ultrasonics, Ferroelectrics & Frequency Control, vol. 43, n. 4, pp. 654-659, July 1996;    An FFT-Based Flow Profiler for High-Resolution In Vivo Investigations, Piero Tortoli et al. Ultrasound in Med. & Biol. Vol. 23 No. 6 pp. 899-910, 1997;    U.S. Pat. No. 6,450,959.    Additional details of the CFM and PW methods may be found in the following documents:    U.S. Pat. No. 4,913,159, U.S. Pat. No. 4,817,618, U.S. Pat. No. 5,724,974 and WO 01/71376.
As shown by the above documents, the disclosures of which are incorporated herein by reference, the above techniques have been long known and widely used. The technology known as Multigate or Multigate Spectral Doppler allows for a quick real-time determination of the spectral profile of Doppler shifts according to the depth of penetration of ultrasonic pulses into the body under examination, with no excessive burden on processing units. The spectra of Doppler frequencies and/or the corresponding velocities are represented as frequencies or corresponding velocities along a first axis and as depths along a second axis of a Cartesian coordinate system.
As clearly shown by the description of the above techniques, all these techniques are based on the Doppler effect and do not allow assessment of the direction of a moving reflector and the direction of the blood flow when the axis of the beam of acoustic pulses, i.e. the direction of propagation thereof, are perpendicular to the displacement direction of the reflector. Indeed, the frequency shift according to the Doppler effect depends from the angle of propagation of a beam impinging against a moving reflector and the function in a cosine which gives a zero factor when the angle of incidence of the beam is 90°.
In PW Doppler in general, and particularly in Multigate technologies, the whole frequency content of the received echo signals corresponding to the back-scattered ultrasound beam by a certain sample volume is determined. When the direction of propagation of ultrasounds is perpendicular to the moving reflector and in a specific case of application of the present invention to blood flow direction, the spectral frequency distribution at the sample volume (gate or depth increment) is symmetric with respect to the line that corresponds to the zero Doppler frequency shift and hence to the zero velocity. On the other hand since in such kind of representation the brightness of the image corresponds to the intensity of the received signal and thus to the number of moving reflectors which are present in a certain sample volume, in the image displayed, in which the Doppler frequency signals of each sample volume are depicted one adjacent to the following sample volume according to the order of sequence of said sample volumes, using the entire frequency content of the Doppler frequency signals at each sample volume provides for a better determination of the sample volumes, in which the flow is considerable, and also the determination of the axis of propagation of the flow. On the other hand when the incidence angle of the beam relative to the flow direction is 90° or approximately 90°, it will not be possible to determine the direction of the flow since the Doppler frequency distribution within the spectrum of the Doppler frequency signal is symmetric being composed of specular positive and negative spectral components.
Multigate technology can be used for simultaneously highlighting and imaging multiple vessels at different penetration depths, i.e. at different distances from the origin of the ultrasonic pulses, within the overall range of penetration depth increments of the Multi-gate process.
In short, with prior art technologies, when the displacement direction of the reflector, and hence of a blood flow, is perpendicular to the direction of the axis of propagation of the incident beam of ultrasonic pulses, neither the CFM method, nor the PW, and particularly the Multi-gate method, allow an assessment of blood flow direction.
Contrary to what might appear, the above condition, in which the displacement direction of the reflector and hence of blood flow, is perpendicular to the direction of the axis of propagation of the incident beam of ultrasonic pulses, is not a rare condition in diagnostic imaging. For example, in hemodynamic imaging of cerebral vessels in the cranium, there are only a few windows through which the ultrasonic beam can be directed to said vessels. Unfortunately, the direction of the ultrasonic pulse beam is often oriented perpendicular to the flow in said vessels.
Furthermore, it is often needed or desired to simultaneously image the flow conditions in adjacent or parallel vessels at different penetration depths of the ultrasonic beam, whereby the direction of the beam axis or the direction of propagation of the acoustic front is fixed and determined by the requirement that all vessels to be imaged must be intersected thereby.
Further difficulties arise if Doppler imaging is used with venous blood flow. Here, vessels have a small size and the venous blood flow is relatively slow. Furthermore blood flux is not constant but varies according to heart cycle and to the inspiration expiration cycle, so that, for each sample volume, blood flow can vary its velocity from a maximum velocity to nearly zero or even to a negative velocity, i.e. to an opposite flow direction. This has the effect that the frequency signal will pass from a maximum value to a value which can be approximately zero or negative and the displayed signal will blink or even change color in the displayed Doppler graph. The variation of the status of appearance of the pixels in the image representing the value of the frequency signals will change from a certain color and brightness when a flux is detected, i.e. when there are moving blood particles to a quite black and/or very low brightness status of appearance when the flux is absent or very slow.
A particular application in which these conditions occur is simultaneous determination of blood flow characteristics in Galen's vein, middle internal cerebral vein and Rosenthal's vein. The determination of venous blood flow in these veins seems to have a considerable clinical and diagnostic relevance for early diagnosis of multiple sclerosis, as reported in Chronic Cerebrospinal Venous Insufficiency in Patients with Multiple Sclerosis, Paolo Zamboni et al., J. Neurol. Neurosurg. Psychiatry, 5 Dec. 2008. Now, in this case the cranial windows through which ultrasonic pulses are transmitted for acoustic treatment of the deep regions containing the veins, the blood flow of which has to be controlled for diagnostic purposes, are such that the above unfavorable condition occurs and the direction of blood flows is not currently detectable.